Generally speaking, infrared photo-conductor sensors essentially provide an electrical response to infrared radiation. Such sensors, therefore, convert infrared radiation into a measurable form (e.g., current, voltage, etc.). The detected energy can be translated into imagery illustrating energy differences between objects, thus enabling perception or identification of an otherwise obscured scene. That is, the electrical response can be measured, digitized, and used to construct an image or an object, scene, or phenomenon that emits the infrared radiation.
Despite not being visibly perceptible to the naked eye, infrared radiation is ever present in our natural environment. In fact, any matter that generates heat emits infrared radiation. Within the spectrum of electromagnetic radiation, infrared radiation has wavelengths ranging from 0.75-3 micron (shortwave infrared)(SWIR), 3-5 micron (midwave infrared)(MWIR), 8-14 micron (longwave infrared)(LWIR), and 14-1000 micron (far infrared).
Infrared photo-conductor sensors can be provided in a variety of general formats including single element-detectors, linear arrays (e.g., a single row of individual detectors/pixels), and two-dimensional focal plane arrays (e.g., multiple rows and columns or individual detectors/pixels).
Many conventional infrared photo-conductor sensors are based on lead-salt sensing technology, such as lead sulfide (PbS), lead selenide (PbSe), and lead telluride (PbTe). Conventional infrared lead-salt focal plane arrays rely on photoconductivity to convert photons into electrical current via free carrier absorption. The infrared lead salt (e.g., PbSe, PbS, PbTe) sensing technology itself was developed in Germany during World War II and is still used as a mid-IR sensors which does not require refrigeration.
However, due to the presence of noise in the form of both Generation-Recombination noise and 1/f noise, substantial signal conditioning is required. The supporting electronics include pre-amplification circuits and electronic chopping, thus requiring the use of large capacitors at each pixel. This approach undesirably complicates the entire system. For example, large pixel sizes of 60×60 microns are used hence limiting the resolution of the array.
In addition, the material characteristics themselves exhibit large variations in uniformity and consistency. The lack of adequate adhesion between the lead-salts and underlying layers is an additional shortcoming to such lead-salt based systems. Chemically-deposited lead-salt films, for instance, suffer from characteristically poor adhesion despite exhibiting, in some circumstances, notably larger response. This lack of adequate adhesion leads to catastrophic film delamination if not during the film preparation, then during the sensor fabrication process (e.g., fabrication of a focal plane array), in which even reasonably low process temperatures of 130° C. have been observed to destroy PbSe films. The poor film adhesion is, in part, subject to delamination because of the large differences in thermal expansion differences between lead-salts (e.g., PbSe—19 ppm/° C.) and, for example, a silicon (2.6 ppm/° C.) substrate. In the area of a single pixel of 2600 μm2 in a conventional structure, for instance, there is minimal adhesion.
Accordingly, there continues to be an industrial need for lead-salt sensing technology having improved properties.